1. Field of the Invention
The present invention relates to a method for quantifying a specific compound, such as glucose, contained in a sample.
2. Description of the Prior Art
As an example of a quantifying method for a specific compound, a method of quantifying glucose will be described below.
In a generally known method of electrochemically quantifying glucose, a glucose oxidase (EC 1.1.3.4) is used in combination with an oxygen electrode or a hydrogen peroxide electrode (described, for example, in "BIOSENSOR", Kodansha, edited by Shuichi Suzuki).
The glucose oxidase selectively oxidizes .beta.-D-glucose, the substrate, into D-glucono-.delta.-lactone using oxygen as an electron acceptor. In this reaction process, the oxygen is reduced to hydrogen peroxide. The quantity of the glucose can be determined by measuring an amount of oxygen consumption with an oxygen electrode or by measuring an amount of generated hydrogen peroxide with a hydrogen peroxide electrode.
According to the above method, as can be inferred from the reaction process, the results of measurement are greatly influenced by the concentration of oxygen dissolved in the sample solution. Furthermore, measurements cannot be made in the absence of oxygen.
In view of the above situation, a new type of glucose sensor has been developed that uses instead of oxygen an organic compound or a metal complex, such as a potassium ferricyanide, ferrocene derivative, quinone derivative, etc., as the electron acceptor. In this type of sensor, the reduced form of the electron acceptor, resulting from the enzymatic reaction, is oxidized with an electrode and the concentration of glucose is determined from its oxidizing current.
Furthermore, when such an electron acceptor is used in place of oxygen, a known amount of glucose oxidase and the electron acceptor can be held on the electrode accurately and in a stable condition. In that case, the electrode system and the reaction layer can be formed in an integral structure in a near dry condition. Disposable glucose sensors based on this technique have been attracting much attention in recent years, since the concentration of glucose can be easily measured just by introducing a test sample into a sensor chip inserted in a measuring device. This method can be applied to the quantification not only of glucose but also of other specific compounds.
The use of the above-mentioned electron acceptors, which is coupled with the technique for forming the electrode system and the reaction layer in an integral structure, has made possible simple electrochemical quantification of various specific compounds. However, in quantifying a specific compound using the above-mentioned method, if there is nonuniformity in the dissolved state of the reaction layer in the test sample, there may occur nonuniformity, for example, in the wetting of the working electrode and counter electrode with respect to the test sample or in the state of an electric double layer formed at the interface between the respective electrode and the substance dissolved in the sample solution, thus causing a potential difference between the two electrodes. This potential difference causes errors or variations in the results of measurement.